In the current assembly and adjusting mechanisms, the outer end of the coil is immobilized by a balance-spring stud fixed to a balance-spring stud holder secured to a balance-cock. An index rotatable relative to the balance-spring stud holder is provided to adjust the active length of the coil. Generally, the index may have two arms: a first arm supporting two pins between which the coil is free, and a second, optional arm serving as a lever to pivot the index around the axis of the balance. When the index pivots, the active length of the coil is reduced or increased.
Other types of devices for assembling and adjusting the coil have been designed, in particular with the aim of simplifying the assembly of the balance spring and adjusting its active length. Patent application WO 2006/102911 A describes one example of this. The device described in this document has a single arm mounted by friction on the balance-cock to put the watch into beat. The arm is provided with a circular groove defining a service position of the terminal coil of the balance spring, at the bottom of which it is kept bearing by a rotating organ, striated over the portion of its periphery arranged bearing against the balance spring. According to this document, the striated rotating organ thus defines the active length of the balance spring, while two pins are also supported by the arm, on either side of the striated rotating organ and near the circular groove, to provide guiding of the terminal coil.
One primary drawback of this device comes from the structure of the organs cooperating with the terminal coil of the balance spring. Indeed, the latter is extremely fragile and delicate, and it its chronometric properties deteriorate very quickly following the mechanical stresses that it may experience. In the case of this device of the prior art, the mechanical stress that the balance spring may experience, due to its interaction with the striated rotating organ, presents a significant risk of deterioration of its qualities. In particular, the rotating organ must necessarily apply a certain pressure on the balance spring to guarantee its driving, which causes damage to the balance spring. Furthermore, it is likely that the balance spring may undergo additional damage through its cooperation with the guide pins. Typically, this type of pin requires a manual intervention, after it is assembled on the arm, to precisely adjust the distance separating it from the bottom of the circular groove. Thus, the periphery of a pin is frequently not exactly parallel to the sides of the balance spring, which may lead to friction during operation, leading to a loss of chronometric precision over time. Lastly, the mounting of the balance spring on such a device already presents a significant risk of deterioration, since it is complex to manipulate during this placement operation.